1. Field of the Invention
Wax containing hydrocarbon mixtures are transported in a conduit by distilling the mixture into an overheads fraction and a bottoms fraction, cooling the overheads fraction, and then mixing it with the hot bottoms fraction to obtain a pumpable slurry.
2. Description of the Prior Art
Pumping viscous hydrocarbon mixtures at temperatures below their pour point is very critical. Heat transfer methods and chemical agents to improve fluid flow properties have been studied. In general, these solutions have proved feasible with low, waxy hydrocarbon mixtures, but not with the more waxy hydrocarbon mixtures. Also, viscous hydrocarbon mixtures have been suspended in water and the combination pumped at temperatures below the pour point of the crude oil.
Patents representative of the prior art include the following:
Kells (U.S. Pat. No. 271,080) separates wax from crude oil by pumping the crude oil in small streams into the bottom of a tank containing a brine at a temperature sufficiently low to congeal the wax. The congealed wax is recovered in the brine.
Coberly (U.S. Pat. No. 2,303,823) teaches pipelining waxy crude oils by blowing cold gas through the oil to form wax crystals and then pumping the mixture at low temperatures.
Scott (U.S. Pat. No. 3,292,647) teaches transporting waxy crude oils in a pipeline by shearing the crude at a temperature below its pour point to break down the wax and form a fine dispersion, then introducing a gas, e.g. N.sub.2, CO.sub.2 and natural gas, into the sheared crude to prevent regrowth of the wax crystals, and thereafter pumping the composition.
Button et al (U.S. Pat. No. 3,393,144) teaches fractionating distillate fuel into a high boiling point fraction (contains a majority of wax) and a low boiling point fraction (contains an insignificant amount of wax), mixing a wax crystal modifier with the high boiling fraction and chilling the mixture to precipitate the wax. The wax particles are then agitated at a low temperature to form a slurry.
Russell and Chapman in J. Inst of Petroleum 57, 117 (1971) teaches the importance of controlled shearing of crude oil during cooling to obtain a pumpable system. The cooling is effected on the composite crude oil by external cooling. With this type of cooling, some of the lower molecular weight hydrocarbons are occluded within the congealed wax particles.
Watanabe (U.S. Pat. No. 3,468,986) forms spherical particles of wax by melting the wax, then dispersing it in a non-solvent liquid, e.g. water, at a temperature above the solidification temperature of the wax, and thereafter cooling the dispersion to solidify the wax droplets.
Titus (U.S. Pat. No. 3,527,692) transports crushed oil shale, slurried in 140-325 mesh solvent, e.g. crude oil, retorted shale oil, or a fraction thereof.
Eagen et al (U.S. Pat. No. 3,681,230) and Gudelis et al (U.S. Pat. No. 3,644,195) dewax oil by contacting the oil with a cold solvent under intense agitation.
Merrill et al (U.S. Pat. No. 3,804,752) separate waxy hydrocarbon mixtures into an overheads fraction and a bottoms fraction, congeal the bottoms fraction into spheres and mix the particles within the overheads fraction to obtain a pumpable slurry.
The art has also used heat, e.g. heat exchangers placed intermittently along the pipeline, to maintain the crude oil above its pour point and thus facilitate pumping. The main disadvantage of this method is that the crude tends to "set up" during shut downs.